A study of mechanochemical activation in solid-state synthesis of advanced ceramic composites

Abstract

Mechanical methods of the activation of chemical processes are currently widely used for the synthesis of various compounds. Intensive (dry) milling of ultra-fine and nanometer-sized powders is considered to be a way of applying “mechanochemical activation” that involves dispersion of solids, generation and migration of defects in the bulk and plastic deformation of particles. Taking into consideration of the possible benefits (such as increasing particle contacts and modifying the structure), such mechanical activation was employed in this study. Using high purity hydrated silicates (kaolinite, talc) and boehmite as precursors, effect of process parameters mainly milling time, milling method and sintering temperature on densification and microstructure of the sintered bodies were investigated. In first series of tests, mechanically induced phase transformations (e.g. formation and transformation of mullite) were observed and studied carefully in differently milled material sintered at temperatures about 1000 °C, 2 h. SEM-EDX analysis and density/porosity measurements showed the differences in grain growth and densification process conditions applied. Formation of specific intermediate phases such as MAS (or μ-cordierite) and the effect of intense mechanical action on phase compositions after sintering were well recognized. Later, experiments were performed at 1300 °C, using the selected milling method from earlier tests, wherein Yttria-stabilised zirconia (YSZ) and ceria were also added as dopants to study the possible effects on sintering behaviour and the resultant microstructure of the composite material. Mechanochemical milling of cordierite precursors mixed with predetermined amounts of YSZ resulted in substantial destabilisation of tetragonal zirconia and formation of zircon after sintering rest time of 2 h. When the additive ratio ceria/YSZ was 1/4, increasing the milling time resulted in peaks indicative of reduced crystallinity in sintered samples. However, in case the ratio was 4, mechanical activation resulted in an increase in the degree of crystallisation in the sintered material, which otherwise (without or with less milling) had a more glassy structure due to the fluxing effect of cerium oxide. Ceria was found to be effective in stabilisation of zirconia and reduction of the transformation to zircon. When ceria was added alone (4 wt%) to the precursor mixture, mechanochemical milling resulted in an increase in the densification and crystallisation of the sintered cordierite. The results of this research could be further investigated an applied for controlled mechanochemical structural modifications based on the required final (physical/chemical) properties of the material in solid-state synthesis of advanced ceramics.

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